Typical examples of liquid crystal cell include a field effect mode cell proposed by M. Schadt et al, APPLIED PHYSICS LETTERS, 18, 127-128 (1971), a dynamic scattering mode cell proposed by G. H. Heilmeier et al, PROCESSING OF THE I.E.E.E., 56, 1162-1171 (1968) and a guest/host mode cell proposed by G. H. Heilmeier et al, APPLIED PHYSICS LETTERS, 13, 91 (1968) or D. L. White et al, JOURNAL OF APPLIED PHYSICS, 45, 4718 (1978).
Among these liquid crystal display cells, TN mode cells, which belong to the field effect mode cells, are majorly used at present. In the case of the TN mode cells, it is required to set the product of the optical anisotropy (.DELTA.n) of the liquid crystal material in the cell and the thickness (d; .mu.m) of the cell to a definite value in order to achieve good cell appearance, as indicated by G. Bauer, Mol. Cryst. Liq. Cryst., 63, 45 (1981). A liquid crystal display cell used in practice has a .DELTA.n.multidot.d value of either 0.5, 1.0, 1.6 or 2.2. Generally speaking, the visual properties of a liquid crystal display cell can be improved by setting the .DELTA.n.multidot.d value to 0.5. On the other hand, the frontal contrast thereof can be improved by setting the .DELTA.n.multidot.d value to 1.0, 1.6 or 2.2. Therefore, it is generally recommended to set the .DELTA.n.multidot.d value of a liquid crystal display cell to 0.5, when it is regarded as important to achieve excellent visual properties from any direction. On the other hand, the .DELTA.n.multidot.d value thereof may be preferably set to 1.0, 1.6 or 2.2 in order to obtain a clear frontal contrast.
On the other hand, the thickness of a liquid crystal cell layer in a practically used liquid crystal display cell is commonly set to a definite value within a limited range of 6 to 10 .mu.m. Thus, a liquid crystal material having a low .DELTA.n value is required in order to set the .DELTA.n.multidot.d value to 0.5. In contrast thereto, a liquid crystal material having a high .DELTA.n value is required in order to set the .DELTA.n.multidot.d value to 1.0, 1.6 or 2.2. Namely, either a liquid crystal material having a low .DELTA.n value or one having a high .DELTA.n value is required depending on the desired display properties.
In the case of the TN mode cells, the dielectric anisotropy (.DELTA..epsilon.) of a mixed liquid crystal is required to be positive. Therefore, it is demanded to provide nematic liquid crystal materials which are large .DELTA..epsilon. values and low threshold voltage and can drive at low voltage.